328 AUSTRALASIAN ANTARCTIC EXPEDITION. 



The oblique extinction of some of the hypersthene suggested that the mineral 

 might be monoclinic pyroxene. The low refractive index and the negative optic 

 character of the mineral differentiated it from augite, and the parallel extinction of 

 cleavage flakes established its orthorhombic nature. Merwin* observed the same 

 phenomenon, and accounted for the oblique extinction by saying that the parting or 

 cleavage developed was parallel to b (010). Doubtless the same explanation holds 

 in this similar instance. 



(c) The Metamorphism of these Rocks. 



The biotite, ilmenite and hornblende of these rocks are the products of the 

 metamorphism of the pyroxene (usually with some inter-action with felspar). Where 

 the granulation of the pyroxene has gone to the stage of the streaking-out into fine 

 granules the production of biotite has been most easily accomplished ; but some of the 

 biotite is associated with the larger masses of pyroxene. It will be remembered that 

 inter-action with felspar has been mentioned above as a necessary part of the formation 

 of the final biotite product from the pyroxene. That this is so is shown by at least 

 two remarkable features of these rocks. The first is the presence of biotite streaks along 

 nearly every crack and cleavage line of some of the felspar grains, and the second is 

 the presence of row upon row of parallel lines of minute dark rectangular and rhombic 

 grains which represent the ultimate effect of the crushing of the original pyroxene. 

 The alternation of regions where these rows are concentrated with bands where the 

 felspathic matter is free from them has developed a microscopic gneissose structure 

 comparable in its essential features with those of macroscopic dimensions. Frequently 

 there is seen a development of biotite associated with these very fine grains. The other 

 streaks of biotite in the felspar where the very fine pyroxene grains are absent represent 

 the conditions which exist when all the content of the pyroxene has been altered (e.g., 

 No. 793). Another feature showing the genetic relationship that exists between the 

 felspar and the biotite is the usual radiating growth of small biotite crystals from a 

 central core of ilmenite. Each of the small biotite crystals penetrates into felspathic 

 material. The effect produced is singular. From the arrangement of these three 

 minerals there can be no doubt that the growths represents a reaction between them. 



4. SUMMARY. 



From this treatment of the pyroxene-plagioclase-gneisses and the hypersthene- 

 alkali-felspar-gneisses it will be seen that the acidity of the rock is the factor which 

 determines whether the metamorphic derivative of the pyroxene shall be biotite or 

 hornblende. In those rocks where the acid-felspar content is relatively low (e.g., No. 

 792) the hornblende-content is great, but where the quartz-felspar-content is great 

 (e.g., No. 778, analysis below) hornblende is absent and the metamorphic derivative is 

 biotite. 



* " The Charnockite Series of Igneous Rocks ": H. S. Washington, Am. Jour. Sci., XLI (1916), p. 331. 



